1. Field of the Invention
The invention relates generally to a method of reducing contaminants in a glass melt, and more specifically to reducing condensation-formed contaminants during a glass stirring process.
2. Technical Background
Chemical and thermal homogeneity is a crucial part of good glass forming operations. The function of a glass melting operation is generally to produce glass with acceptable levels of gaseous or solid inclusions, but this glass usually has cord (or striae or ream) of chemically dissimilar phases. These non-homogeneous components of the glass result from a variety of normal occurrences during the melting process including refractory dissolution, melting stratification, glass surface volatilization, and temperature differences. The resulting cords are visible in the glass because of color and/or index differences.
One approach for improving the homogeneity of glass is to pass the molten glass through a vertically-oriented stirring apparatus located downstream of the melter. Such stirring apparatus are equipped with a stirrer having a central shaft rotated by a suitable driving force, such as a motor. A plurality of blades extends from the shaft and mix the molten glass as it passes from the top to the bottom of the stirring apparatus. The operation of such stir chambers should not introduce further defects into the resulting glass, specifically, defects arising from condensed oxides.
Volatile oxides in a glass stirring apparatus can be formed from any of the elements present in the glass and stirring apparatus. Some of the most volatile and damaging oxides are formed from Pt, As, Sb, B, and Sn. Primary sources of condensable oxides in a glass melt include hot platinum surfaces for PtO2, and the glass free surface for B2O3, As4O6, Sb4O6, and SnO2. By glass free surface what is meant is the surface of the glass which is exposed to the atmosphere within the stirring apparatus. Because the atmosphere above the glass free surface, and which atmosphere may contain any or all of the foregoing, or other volatile materials, is hotter than the atmosphere outside of the stirring apparatus, there is a natural tendency for the atmosphere above the free glass surface to flow upward through any opening, such as through the annular space between the stirrer shaft and the stirring vessel cover. Since the stirrer shaft becomes cooler as the distance between the stirrer shaft and the glass free surface increases, the volatile oxides contained with the stirring apparatus atmosphere can condense onto the surface of the shaft if the shaft and/or cover temperature are below the dew point of the oxides. When the resulting condensates reach a critical size they can break off, falling into the glass and causing inclusion or blister defects in the glass product.
Heating the shaft above the glass free surface has proven only partially successful in reducing particulate contamination in the glass melt, resulting only in a stratification of the condensation.
One prior art method of reducing contamination of the glass melt by condensates has been to dispose a disc-shaped shield between the glass free surface and upper portions of the stir chamber. However, such methods may make it difficult to control the temperature of the glass free surface, such as by heating the chamber cover above the glass. In addition, the joint between the shield and the stirrer shaft may serve as an additional source of condensate contamination.